edit..oh duh.. SORRY.....I just reread and.I see you sprayed it with soap.....figures...soap is bad for leaves, never heard it hurting stems, but maybe when you wiped it off, it made it worse..(see below)
Doesn't surprise me though....salvias are sensitive plants, and test sprays are a good idea, starting with half strengths on leaves, before going to full strength. Many large leaved shade plants can't tolerate soap products no matter what, and oils are used.
Ok heres my original post, which is good advice anyways!!!

could have been from handling stem, when square edges of stem are damaged, integrity of stem is compromised, and this problem can ensue....always handle those corners with utmost caution, especially with fresh cuttings in the kitchen or garden, as mucho germs abound in those areas....use sterile tools, and rinse between cuts etc if you can.
leaves and nodes above that browning section will die soon, (unless there are any connective strips of green tissue??) so if there are any nodes above, better take a cutting now.....cause these sorts of things never reverse themselves

usually stems do that from the top down, without green above, its quite natural with cuttings and trasplants, newgrowth will sprout from below......so its odd to see it in the middle, (which is why I think you accidentally damaged the edges...there are some virus;s which do this, but they rarely hit salivas.....black rot is soft and gushy, and can hit with damage like that as well, but plant just falls right over.....